1. Field of the Invention
The invention relates to a means of controlling a radial path of resistance in a semiconductor rod and somewhat more particularly to a method and apparatus for controlling the radial path of resistance in a semiconductor rod undergoing a crucible-free zone melt treatment wherein a semiconductor rod, such as composed of silicon, is vertically supported at its ends so that an inductive heating means positioned to surround an axial portion of the rod produces a melt zone in the surrounded portion and a protective gas atmosphere is flown past the melt zone while the melt zone is moved axially along the rod to produce a monocrystalline rod having a selected radial path of resistance through a cross-section thereof.
2. Prior Art
Production of monocrystalline rods by a crucible-free zone melting process is known wherein polycrystalline semiconductor rods, particularly silicon rods, are transformed, with the aid of a seed crystal, into monocrystalline rods by moving a melt zone from the end of the polycrystalline rod attached to the seed crystal to the other end of the stock semiconductor rod. In this arrangement, the semiconductor rod is generally vertically supported by opposing rod holding means, at least one of which is rotated about the rod axis during the process so that a symmetrical growth of the re-solidified material is attained.
It is generally desirable to produce monocrystalline rods for use in semiconductor components which have very uniform values of resistance throughout their cross-sections, i.e. a uniform radial path of resistance. In other words, it is desirable in the production of monocrystalline rods to very thoroughly mix the matter within the melt zone to enable dopants, etc. to be uniformly incorporated at all points of the resultant crystal lattice.
For use in producing semiconductor components, such as overhead ignitable thyristors, a basic semiconductor material is utilized which provides (111) orientated silicon crystal discs having a break or discontinuity in the electric specific resistance at the center of such a disc.
A process of producing such basic semiconductor materials is known. For example, German Offenlegungsschrift 2,204,486 or British Patent Specification 1,362,739 suggests that the radial path of resistance is controlled by supplying axially directed energy to one of the rod holding means which transmits the energy to the melt zone for mixing of matter therein. In this process, energy is transferred to the melt zone via current or sonic pulses.
Other uses of semiconductor material require a uniformly radial path of resistance without a center break and other processes may be used to produce such basic materials whereby control of the melt zone in terms of temperature distribution and material flow pattern is obtained, which in turn influences the mixing of materials therein. Thus, for example, German Pat. No. 1,218,404 discloses a process of crucible-free zone melting of a semiconductor rod in which a mutual lateral displacement of the rod holding means relative to the heating device produces an additional agitation effect in the melt zone. This agitation functions to more or less uniformly distribute dopants etc. within the recrystallized rod.